Homo erectus was not alone in ancient Africa. Newly discovered fossil evidence, detailed in the latest issue of Nature, strongly suggests that no fewer than three distinct species of early humans from the genus Homo co-existed on the continent between 1.7 and 2 million years ago.

The findings, which run counter to the hypothesis that modern humans evolved linearly out of H. erectus, provide some of the most compelling evidence to date that there were multiple, parallel lines of evolution early in our genus' history.

This groundbreaking find came in the form of three fossils: a well preserved skull, complete with a flat face; an almost complete lower jaw, perhaps the most well-preserved specimen of its kind ever discovered; and a fragment of another lower jaw, all discovered in the deserts of Northern Kenya. Together, the specimens complement a mysterious cranium fossil first discovered all the way back in 1972. At the time, the specimen (which was large, and featured a flat face unlike any associated with H. erectus) was determined to belong to an entirely different species, Homo rudolfensis, but the absence of a lower jaw that matches with that of the specimen has made justifying this taxonomical distinction somewhat difficult.

The three newly discovered fossils have made that task much more manageable. The skull, thought to belong to a juvenile, bears what first author Meave Leakey and her colleagues believe is a strong anatomical resemblance to the one discovered in 1972, as do the lower jaws. The more complete of the two jaws is pictured here, and above in combination with the cranium fossil from 1972.

Taken together, Leakey writes that the newly discovered trio of specimens "clarify the anatomy and taxonomic status of" the 1972 cranium, classified as Homo rudolfensis, and "confirm the presence of two contemporary species of early Homo [i.e. H. rudolfensis and H. habilis], in addition to Homo erectus, in the early Pleistocene of eastern Africa."

By extension, these specimens play a critical role in settling a long-standing debate over modern humans' evolutionary origins.

"This new material certainly substantiates the idea, long gathering ground, that multiple lineages of early Homo are present in the record at Koobi Fora," explained Ian Tattersall of the American Museum of Natural History in an interview with the New York Times. He continues:

"And it supports the view that the early history of Homo involved vigorous experimentation with the biological and behavioral potential of the new genus, instead of a slow process of refinement in a central lineage."